1
|
Maltsev AV, Stern MD, Maltsev VA. Disorder in Ca2+ release unit locations confers robustness but cuts flexibility of heart pacemaking. J Gen Physiol 2022; 154:e202113061. [PMID: 35943725 PMCID: PMC9366202 DOI: 10.1085/jgp.202113061] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2021] [Revised: 05/04/2022] [Accepted: 06/21/2022] [Indexed: 12/12/2022] Open
Abstract
Excitation-contraction coupling kinetics is dictated by the action potential rate of sinoatrial-nodal cells. These cells generate local Ca releases (LCRs) that activate Na/Ca exchanger current, which accelerates diastolic depolarization and determines the pace. LCRs are generated by clusters of ryanodine receptors, Ca release units (CRUs), residing in the sarcoplasmic reticulum. While CRU distribution exhibits substantial heterogeneity, its functional importance remains unknown. Using numerical modeling, here we show that with a square lattice distribution of CRUs, Ca-induced-Ca-release propagation during diastolic depolarization is insufficient for pacemaking within a broad range of realistic ICaL densities. Allowing each CRU to deviate randomly from its lattice position allows sparks to propagate, as observed experimentally. As disorder increases, the CRU distribution exhibits larger empty spaces and simultaneously CRU clusters, as in Poisson clumping. Propagating within the clusters, Ca release becomes synchronized, increasing action potential rate and reviving pacemaker function of dormant/nonfiring cells. However, cells with fully disordered CRU positions could not reach low firing rates and their β-adrenergic-receptor stimulation effect was substantially decreased. Inclusion of Cav1.3, a low-voltage activation L-type Ca channel isoform into ICaL, strongly increases recruitment of CRUs to fire during diastolic depolarization, increasing robustness of pacemaking and complementing effects of CRU distribution. Thus, order/disorder in CRU locations along with Cav1.3 expression regulates pacemaker function via synchronization of CRU firing. Excessive CRU disorder and/or overexpression of Cav1.3 boosts pacemaker function in the basal state, but limits the rate range, which may contribute to heart rate range decline with age and disease.
Collapse
Affiliation(s)
- Anna V. Maltsev
- School of Mathematics, Queen Mary University of London, London, UK
| | - Michael D. Stern
- Laboratory of Cardiovascular Science, Intramural Research Program, National Institute on Aging, National Institutes of Health, Baltimore, MD
| | - Victor A. Maltsev
- Laboratory of Cardiovascular Science, Intramural Research Program, National Institute on Aging, National Institutes of Health, Baltimore, MD
| |
Collapse
|
2
|
Regulation of sinoatrial funny channels by cyclic nucleotides: From adrenaline and I K2 to direct binding of ligands to protein subunits. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2021; 166:12-21. [PMID: 34237319 DOI: 10.1016/j.pbiomolbio.2021.06.010] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Subscribe] [Scholar Register] [Received: 02/11/2021] [Revised: 06/13/2021] [Accepted: 06/25/2021] [Indexed: 12/27/2022]
Abstract
The funny current, and the HCN channels that form it, are affected by the direct binding of cyclic nucleotides. Binding of these second messengers causes a depolarizing shift of the activation curve, which leads to greater availability of current at physiological membrane voltages. This review outlines a brief history on this regulation and provides some evidence that other cyclic nucleotides, especially cGMP, may be important for the regulation of the funny channel in the heart. Current understanding of the molecular mechanism of cyclic nucleotide regulation is also presented, which includes the notions that full and partial agonism occur as a consequence of negatively cooperative binding. Knowledge gaps, including a potential role of cyclic nucleotide-regulation of the funny current under pathophysiological conditions, are included. The work highlighted here is in dedication to Dario DiFrancesco on his retirement.
Collapse
|
3
|
Bidaud I, D'Souza A, Forte G, Torre E, Greuet D, Thirard S, Anderson C, Chung You Chong A, Torrente AG, Roussel J, Wickman K, Boyett MR, Mangoni ME, Mesirca P. Genetic Ablation of G Protein-Gated Inwardly Rectifying K + Channels Prevents Training-Induced Sinus Bradycardia. Front Physiol 2021; 11:519382. [PMID: 33551824 PMCID: PMC7857143 DOI: 10.3389/fphys.2020.519382] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2019] [Accepted: 12/17/2020] [Indexed: 11/13/2022] Open
Abstract
Background: Endurance athletes are prone to bradyarrhythmias, which in the long-term may underscore the increased incidence of pacemaker implantation reported in this population. Our previous work in rodent models has shown training-induced sinus bradycardia to be due to microRNA (miR)-mediated transcriptional remodeling of the HCN4 channel, leading to a reduction of the "funny" (I f) current in the sinoatrial node (SAN). Objective: To test if genetic ablation of G-protein-gated inwardly rectifying potassium channel, also known as I KACh channels prevents sinus bradycardia induced by intensive exercise training in mice. Methods: Control wild-type (WT) and mice lacking GIRK4 (Girk4 -/-), an integral subunit of I KACh were assigned to trained or sedentary groups. Mice in the trained group underwent 1-h exercise swimming twice a day for 28 days, 7 days per week. We performed electrocardiogram recordings and echocardiography in both groups at baseline, during and after the training period. At training cessation, mice were euthanized and SAN tissues were isolated for patch clamp recordings in isolated SAN cells and molecular profiling by quantitative PCR (qPCR) and western blotting. Results: At swimming cessation trained WT mice presented with a significantly lower resting HR that was reversible by acute I KACh block whereas Girk4 -/- mice failed to develop a training-induced sinus bradycardia. In line with HR reduction, action potential rate, density of I f, as well as of T- and L-type Ca2+ currents (I CaT and I CaL ) were significantly reduced only in SAN cells obtained from WT-trained mice. I f reduction in WT mice was concomitant with downregulation of HCN4 transcript and protein, attributable to increased expression of corresponding repressor microRNAs (miRs) whereas reduced I CaL in WT mice was associated with reduced Cav1.3 protein levels. Strikingly, I KACh ablation suppressed all training-induced molecular remodeling observed in WT mice. Conclusion: Genetic ablation of cardiac I KACh in mice prevents exercise-induced sinus bradycardia by suppressing training induced remodeling of inward currents I f, I CaT and I CaL due in part to the prevention of miR-mediated transcriptional remodeling of HCN4 and likely post transcriptional remodeling of Cav1.3. Strategies targeting cardiac I KACh may therefore represent an alternative to pacemaker implantation for bradyarrhythmias seen in some veteran athletes.
Collapse
Affiliation(s)
- Isabelle Bidaud
- Institut de Génomique Fonctionnelle, Université de Montpellier, CNRS, INSERM, Montpellier, France.,LabEx Ion Channels Science and Therapeutics, Montpellier, France
| | - Alicia D'Souza
- Division of Cardiovascular Sciences, University of Manchester, Manchester, United Kingdom
| | - Gabriella Forte
- Division of Cardiovascular Sciences, University of Manchester, Manchester, United Kingdom
| | - Eleonora Torre
- Institut de Génomique Fonctionnelle, Université de Montpellier, CNRS, INSERM, Montpellier, France.,LabEx Ion Channels Science and Therapeutics, Montpellier, France
| | - Denis Greuet
- Institut de Génomique Fonctionnelle, Université de Montpellier, CNRS, INSERM, Montpellier, France
| | - Steeve Thirard
- Institut de Génomique Fonctionnelle, Université de Montpellier, CNRS, INSERM, Montpellier, France
| | - Cali Anderson
- Division of Cardiovascular Sciences, University of Manchester, Manchester, United Kingdom
| | - Antony Chung You Chong
- Institut de Génomique Fonctionnelle, Université de Montpellier, CNRS, INSERM, Montpellier, France.,LabEx Ion Channels Science and Therapeutics, Montpellier, France
| | - Angelo G Torrente
- Institut de Génomique Fonctionnelle, Université de Montpellier, CNRS, INSERM, Montpellier, France.,LabEx Ion Channels Science and Therapeutics, Montpellier, France
| | - Julien Roussel
- Institut de Génomique Fonctionnelle, Université de Montpellier, CNRS, INSERM, Montpellier, France
| | - Kevin Wickman
- Department of Pharmacology, University of Minnesota, Minneapolis, MN, United States
| | - Mark R Boyett
- Division of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Matteo E Mangoni
- Institut de Génomique Fonctionnelle, Université de Montpellier, CNRS, INSERM, Montpellier, France.,LabEx Ion Channels Science and Therapeutics, Montpellier, France
| | - Pietro Mesirca
- Institut de Génomique Fonctionnelle, Université de Montpellier, CNRS, INSERM, Montpellier, France.,LabEx Ion Channels Science and Therapeutics, Montpellier, France
| |
Collapse
|
4
|
Abstract
Cardiac pacemaking is a most fundamental cardiac function, thoroughly investigated for decades with a multiscale approach at organ, tissue, cell and molecular levels, to clarify the basic mechanisms underlying generation and control of cardiac rhythm. Understanding the processes involved in pacemaker activity is of paramount importance for a basic physiological knowledge, but also as a way to reveal details of pathological dysfunctions useful in the perspective of a therapeutic approach. Among the mechanisms involved in pacemaking, the "funny" (If) current has properties most specifically fitting the requirements for generation and control of repetitive activity, and has consequently received the most attention in studies of the pacemaker function. Present knowledge of the basic mechanisms of pacemaking and the properties of funny channels has led to important developments of clinical relevance. These include: (1) the successful development of heart rate-reducing agents, such as ivabradine, able to control cardiac rhythm and useful in the treatment of diseases such as coronary artery disease, heart failure and tachyarrhythmias; (2) the understanding of the genetic basis of disorders of cardiac rhythm caused by HCN channelopathies; (3) the design of strategies to implement biological pacemakers based on transfer of HCN channels or of stem cell-derived pacemaker cells expressing If, with the ultimate goal to replace electronic devices. In this review, I will give a brief historical account of the discovery of the funny current and the development of the concept of If-based pacemaking, in the context of a wider, more complex model of cardiac rhythmic function.
Collapse
Affiliation(s)
- Dario DiFrancesco
- Department of Biosciences, University of Milano, IBF-CNR University of Milano Unit, Milan, Italy
| |
Collapse
|
5
|
Bagliani G, Leonelli F, Padeletti L. P Wave and the Substrates of Arrhythmias Originating in the Atria. Card Electrophysiol Clin 2017; 9:365-382. [PMID: 28838546 DOI: 10.1016/j.ccep.2017.05.001] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The sinus node is the primary cardiac pacemaker from which the wavefront of activation proceeds through bundles of atrial fibers to the atrioventricular node. Left atrial activation proceeds along the Bachmann bundle and lower right atrium, determining P-wave morphology. Electrocardiogram reveals ectopic or retrograde atrial activation, wandering pacemaker activity, or artificial pacemaker-mediated atrial depolarization. Vectorcardiography and transesophageal recording are complementary methods. Atrial anatomic structure and automatic cells outside the sinus node constitute the mechanisms of focal and reentrant atrial arrhythmias. Arrhythmias with specific arrhythmogenic mechanisms correspond to precise electrocardiographic morphology for accurate diagnosis.
Collapse
Affiliation(s)
- Giuseppe Bagliani
- Arrhythmology Unit, Cardiology Department, Foligno General Hospital, Via Massimo Arcamone, 06034 Foligno (PG), Italy; Cardiovascular Diseases Department, University of Perugia, Piazza Menghini 1, 06129 Perugia, Italy.
| | - Fabio Leonelli
- Cardiology Department James A. Haley Veterans' Hospital, University South Florida, 13000 Bruce B Down Boulevard, Tampa, FL 33612, USA
| | - Luigi Padeletti
- Heart and Vessels Department, University of Florence, Largo Brambilla, 3, 50134 Florence, Italy; IRCCS Multimedica, Cardiology Department, Via Milanese, 300, 20099 Sesto San Giovanni, Italy
| |
Collapse
|
6
|
Bai X, Wang K, Yuan Y, Li Q, Dobrzynski H, Boyett MR, Hancox JC, Zhang H. Mechanism underlying impaired cardiac pacemaking rhythm during ischemia: A simulation study. CHAOS (WOODBURY, N.Y.) 2017; 27:093934. [PMID: 28964153 DOI: 10.1063/1.5002664] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Ischemia in the heart impairs function of the cardiac pacemaker, the sinoatrial node (SAN). However, the ionic mechanisms underlying the ischemia-induced dysfunction of the SAN remain elusive. In order to investigate the ionic mechanisms by which ischemia causes SAN dysfunction, action potential models of rabbit SAN and atrial cells were modified to incorporate extant experimental data of ischemia-induced changes to membrane ion channels and intracellular ion homeostasis. The cell models were incorporated into an anatomically detailed 2D model of the intact SAN-atrium. Using the multi-scale models, the functional impact of ischemia-induced electrical alterations on cardiac pacemaking action potentials (APs) and their conduction was investigated. The effects of vagal tone activity on the regulation of cardiac pacemaker activity in control and ischemic conditions were also investigated. The simulation results showed that at the cellular level ischemia slowed the SAN pacemaking rate, which was mainly attributable to the altered Na+-Ca2+ exchange current and the ATP-sensitive potassium current. In the 2D SAN-atrium tissue model, ischemia slowed down both the pacemaking rate and the conduction velocity of APs into the surrounding atrial tissue. Simulated vagal nerve activity, including the actions of acetylcholine in the model, amplified the effects of ischemia, leading to possible SAN arrest and/or conduction exit block, which are major features of the sick sinus syndrome. In conclusion, this study provides novel insights into understanding the mechanisms by which ischemia alters SAN function, identifying specific conductances as contributors to bradycardia and conduction block.
Collapse
Affiliation(s)
- Xiangyun Bai
- School of Computer Science and Technology, Harbin Institute of Technology, Harbin 150001, China
| | - Kuanquan Wang
- School of Computer Science and Technology, Harbin Institute of Technology, Harbin 150001, China
| | - Yongfeng Yuan
- School of Computer Science and Technology, Harbin Institute of Technology, Harbin 150001, China
| | - Qince Li
- School of Computer Science and Technology, Harbin Institute of Technology, Harbin 150001, China
| | - Halina Dobrzynski
- Institute of Cardiovascular Sciences, The University of Manchester, M13 9PL Manchester, United Kingdom
| | - Mark R Boyett
- Institute of Cardiovascular Sciences, The University of Manchester, M13 9PL Manchester, United Kingdom
| | - Jules C Hancox
- Biological Physics Group, School of Physics and Astronomy, The University of Manchester, M13 9PL Manchester, United Kingdom
| | - Henggui Zhang
- School of Computer Science and Technology, Harbin Institute of Technology, Harbin 150001, China
| |
Collapse
|
7
|
Hanna P, Rajendran PS, Ajijola OA, Vaseghi M, Andrew Armour J, Ardell JL, Shivkumar K. Cardiac neuroanatomy - Imaging nerves to define functional control. Auton Neurosci 2017; 207:48-58. [PMID: 28802636 DOI: 10.1016/j.autneu.2017.07.008] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2017] [Revised: 07/22/2017] [Accepted: 07/28/2017] [Indexed: 01/08/2023]
Abstract
The autonomic nervous system regulates normal cardiovascular function and plays a critical role in the pathophysiology of cardiovascular disease. Further understanding of the interplay between the autonomic nervous system and cardiovascular system holds promise for the development of neuroscience-based cardiovascular therapeutics. To this end, techniques to image myocardial innervation will help provide a basis for understanding the fundamental underpinnings of cardiac neural control. In this review, we detail the evolution of gross and microscopic anatomical studies for functional mapping of cardiac neuroanatomy.
Collapse
Affiliation(s)
- Peter Hanna
- University of California Los Angeles (UCLA) Cardiac Arrhythmia Center and Neurocardiology Research Center of Excellence, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA
| | - Pradeep S Rajendran
- University of California Los Angeles (UCLA) Cardiac Arrhythmia Center and Neurocardiology Research Center of Excellence, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA; Molecular, Cellular & Integrative Physiology Program, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA
| | - Olujimi A Ajijola
- University of California Los Angeles (UCLA) Cardiac Arrhythmia Center and Neurocardiology Research Center of Excellence, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA
| | - Marmar Vaseghi
- University of California Los Angeles (UCLA) Cardiac Arrhythmia Center and Neurocardiology Research Center of Excellence, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA
| | - J Andrew Armour
- University of California Los Angeles (UCLA) Cardiac Arrhythmia Center and Neurocardiology Research Center of Excellence, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA
| | - Jefrrey L Ardell
- University of California Los Angeles (UCLA) Cardiac Arrhythmia Center and Neurocardiology Research Center of Excellence, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA; Molecular, Cellular & Integrative Physiology Program, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA
| | - Kalyanam Shivkumar
- University of California Los Angeles (UCLA) Cardiac Arrhythmia Center and Neurocardiology Research Center of Excellence, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA; Molecular, Cellular & Integrative Physiology Program, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA.
| |
Collapse
|
8
|
Kullmann PHM, Sikora KM, Clark KL, Arduini I, Springer MG, Horn JP. HCN hyperpolarization-activated cation channels strengthen virtual nicotinic EPSPs and thereby elevate synaptic amplification in rat sympathetic neurons. J Neurophysiol 2016; 116:438-47. [PMID: 27146984 DOI: 10.1152/jn.00223.2016] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2016] [Accepted: 05/02/2016] [Indexed: 12/27/2022] Open
Abstract
The influence of hyperpolarization-activated cation current (h-current; Ih) upon synaptic integration in paravertebral sympathetic neurons was studied together with expression of hyperpolarization-activated cyclic nucleotide-gated (HCN) subunit isoforms. All four HCN subunits were detected in homogenates of the rat superior cervical ganglion (SCG) using the PCR to amplify reverse-transcribed messenger RNAs (RT-PCR) and using quantitative PCR. Voltage clamp recordings from dissociated SCG neurons at 35°C detected Ih in all cells, with a maximum hyperpolarization-activated cation conductance of 1.2 ± 0.1 nS, half-maximal activation at -87.6 mV, and reversal potential of -31.6 mV. Interaction between Ih and synaptic potentials was tested with virtual fast nicotinic excitatory postsynaptic potentials (EPSPs) created with dynamic clamp. The blocking of Ih with 15 μM ZD7288 hyperpolarized cells by 4.7 mV and increased the virtual synaptic conductance required to stimulate an action potential from 7.0 ± 0.9 nS to 12.1 ± 0.9 nS. In response to stimulation with 40 s long trains of virtual EPSPs, ZD7288 reduced postsynaptic firing from 2.2 to 1.7 Hz and the associated synaptic amplification from 2.2 ± 0.1 to 1.7 ± 0.2. Cyclic nucleotide binding to HCN channels was simulated by blocking native Ih with ZD7288, followed by reconstitution with virtual Ih using a dynamic clamp model of the voltage clamp data. Over a 30-mV range, shifting the half-activation voltage for Ih in 10 mV depolarizing increments always increased synaptic gain. These results indicate that Ih, in sympathetic neurons, can strengthen nicotinic EPSPs and increase synaptic amplification, while also working as a substrate for cyclic nucleotide-dependent modulation.
Collapse
Affiliation(s)
- Paul H M Kullmann
- Department of Neurobiology and Center for Neuroscience, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| | - Kristine M Sikora
- Department of Neurobiology and Center for Neuroscience, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| | - K Lyles Clark
- Department of Neurobiology and Center for Neuroscience, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| | - Irene Arduini
- Department of Neurobiology and Center for Neuroscience, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| | - Mitchell G Springer
- Department of Neurobiology and Center for Neuroscience, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| | - John P Horn
- Department of Neurobiology and Center for Neuroscience, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| |
Collapse
|
9
|
Baruscotti M, Bianco E, Bucchi A, DiFrancesco D. Current understanding of the pathophysiological mechanisms responsible for inappropriate sinus tachycardia: role of the If "funny" current. J Interv Card Electrophysiol 2016; 46:19-28. [PMID: 26781742 DOI: 10.1007/s10840-015-0097-y] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/10/2015] [Accepted: 12/28/2015] [Indexed: 01/01/2023]
Abstract
BACKGROUND Together with the afferent branches of the autonomic nervous system, the sinoatrial node (SAN) forms a functional unit whose function is to fire rhythmic action potentials at a rate optimal for coping with the metabolic needs of the body. Dysfunctional behavior of this complex unit may thus result in SAN rhythm disorders. Among these disorders, there is the inappropriate sinus tachycardia (IST) which occurs when an unjustified fast SAN rate is present. METHODS We here present a critical review of the role of pacemaker f/HCN channels in cardiac rhythm generation and modulation and their involvement in IST. RESULTS Recent evidence demonstrates that a familial form of IST is associated with a gain-of-function mutation in the HCN4 pacemaker channel (R524Q) which confers an increased sensitivity to the second messenger cAMP, a key mediator in sympathetic modulation. CONCLUSIONS This finding is consistent with the general view that hypersympathetic tone is one of the causes of IST and introduces the novel concept of defective funny channel-dependent tachyarrhythmias.
Collapse
Affiliation(s)
- Mirko Baruscotti
- Department of Biosciences, Centro Interuniversitario di Medicina Molecolare e Biofisica Applicata, University of Milano, via Celoria 26, 20133, Milano, Italy.
| | - Elisabetta Bianco
- Cardiovascular Department, "Ospedali Riuniti di Trieste", University Hospital, Trieste, Italy
| | - Annalisa Bucchi
- Department of Biosciences, Centro Interuniversitario di Medicina Molecolare e Biofisica Applicata, University of Milano, via Celoria 26, 20133, Milano, Italy
| | - Dario DiFrancesco
- Department of Biosciences, Centro Interuniversitario di Medicina Molecolare e Biofisica Applicata, University of Milano, via Celoria 26, 20133, Milano, Italy.
| |
Collapse
|
10
|
Chaudhary R, Garg J, Krishnamoorthy P, Shah N, Lanier G, Martinez MW, Freudenberger R. Ivabradine: Heart Failure and Beyond. J Cardiovasc Pharmacol Ther 2015; 21:335-43. [PMID: 26721645 DOI: 10.1177/1074248415624157] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/15/2015] [Accepted: 11/16/2015] [Indexed: 11/16/2022]
Abstract
Heart failure affects over 5 million people in the United States and carries a high rate of mortality. Ivabradine, a new agent has been added to the current medical options for managing heart failure. It is a selective funny current (If) inhibitor in sinoatrial node and slows its firing rate, prolonging diastolic depolarization without a negative inotropic effect. Ivabradine was only recently approved by Food and Drug administration after the results of Systolic Heart Failure Treatment with the If Inhibitor Ivabradine (SHIFT) trial, for a reduction in rehospitalizations from chronic heart failure. This trial assessed patients with stable heart failure with reduced ejection fraction and a heart rate of at least 70 beats per minute at rest on maximally tolerated beta-blocker therapy and demonstrated statistically significant reduction in heart failure hospitalization and deaths. Additionally, ivabradine has been associated with reduced cardiac remodeling, reduced heart rate variability, improvement in exercise tolerance, improved heart failure class of New York Heart Association, and better quality of life. It has also been tried in other conditions, such as inappropriate sinus tachycardia and cardiogenic shock, and is currently in phase II trial for patients with newly diagnosed multiple organ dysfunction syndrome.
Collapse
Affiliation(s)
- Rahul Chaudhary
- Department of Medicine, Sinai Hospital of Baltimore, Johns Hopkins University, Baltimore, MD, USA
| | - Jalaj Garg
- Division of Cardiology, Lehigh Valley Health Network, Allentown, PA, USA
| | - Parasuram Krishnamoorthy
- Department of Medicine, Englewood Hospital and Medical Center, Mount Sinai School of Medicine, Englewood, NJ, USA
| | - Neeraj Shah
- Division of Cardiology, Lehigh Valley Health Network, Allentown, PA, USA
| | - Gregg Lanier
- Division of Cardiology, Westchester Medical Center, New York Medical College, Valhalla, NY, USA
| | - Mathew W Martinez
- Division of Cardiology, Lehigh Valley Health Network, Allentown, PA, USA
| | | |
Collapse
|
11
|
Fang YB, Liu X, Wen J, Tang XJ, Yu FX, Deng MB, Wu CX, Liao B. Differentiation induction of mouse cardiac stem cells into sinus node-like cells by co-culturing with sinus node. INTERNATIONAL JOURNAL OF CLINICAL AND EXPERIMENTAL PATHOLOGY 2014; 7:1868-1879. [PMID: 24966897 PMCID: PMC4069922] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Subscribe] [Scholar Register] [Received: 01/12/2014] [Accepted: 02/27/2014] [Indexed: 06/03/2023]
Abstract
Sinus nodal cells can generate a diastolic or "pacemaker" depolarization at the end of an action potential driving the membrane potential slowly up to the threshold for firing the next action potential. It has been proved that adult cardiac stem cells (CSCs) can differentiate into sinus nodal cells by demethylating agent. However, there is no report about adult CSCs-derived sinus nodal cells with pacemaker current (the funny current, I f). In this study, we isolated the mouse adult CSCs from mouse hearts by the method of tissue explants adherence. The expression of c-kit protein indicated the isolation of CSCs. Then we co-cultured mouse CSCs with mouse sinus node tissue to induce the differentiation of these CSCs into sinus node-like cells, which was proved by identifying the enhanced expression of marker proteins cTnI, cTnT and α-Actinin with Immunofluorescence staining. At the same time, with whole-cell patch-clamp we detected the I f current, which can be blocked by CsCl, in these differentiated cells. In conclusion, by confirming specific I f current in the induced node-like cells, our work shows a method inducing differentiation of CSCs into sinus node-like cells, which can provide helpful information for the further research on sick sinus syndrome.
Collapse
Affiliation(s)
- Yi-Bing Fang
- Department of Cardiothoracic, The Affiliated Hospital of Luzhou Medical CollegeSichuan, P. R. China
| | - Xuan Liu
- Department of Cardiothoracic, The Affiliated Hospital of Luzhou Medical CollegeSichuan, P. R. China
| | - Jing Wen
- Department of Electrophysiology, Institute of Cardiovasology, Luzhou Medical CollegeSichuan, P. R. China
| | - Xiao-Jun Tang
- Department of Cardiothoracic, The Affiliated Hospital of Luzhou Medical CollegeSichuan, P. R. China
| | - Feng-Xu Yu
- Department of Cardiothoracic, The Affiliated Hospital of Luzhou Medical CollegeSichuan, P. R. China
| | - Ming-Bin Deng
- Department of Cardiothoracic, The Affiliated Hospital of Luzhou Medical CollegeSichuan, P. R. China
| | - Chang-Xue Wu
- Department of Cardiothoracic, The Affiliated Hospital of Luzhou Medical CollegeSichuan, P. R. China
| | - Bin Liao
- Department of Cardiothoracic, The Affiliated Hospital of Luzhou Medical CollegeSichuan, P. R. China
| |
Collapse
|
12
|
Rosa GM, Ferrero S, Ghione P, Valbusa A, Brunelli C. An evaluation of the pharmacokinetics and pharmacodynamics of ivabradine for the treatment of heart failure. Expert Opin Drug Metab Toxicol 2013; 10:279-91. [PMID: 24377458 DOI: 10.1517/17425255.2014.876005] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
INTRODUCTION Ivabradine is a new heart-rate-lowering drug; the aim of this review was to analyze its role in heart failure (HF). AREAS COVERED This systematic review on the role of ivabradine in HF is based on material searched and obtained through Pubmed and Medline up to September 2013. EXPERT OPINION Heart rate (HR) is a risk factor in patients with HF, and its reduction is considered an important goal of therapy. The BEAUTIFUL trial demonstrated the benefits of ivabradine on prognosis (only on ischemic endpoints) in patients with coronary artery disease (CAD) and left ventricular systolic dysfunction (LVSD) and HR ≥ 60 bpm. In the SHIFT trial, which enrolled patients with LVSD, HF and HR ≥ 70 bpm, ivabradine administration (on top of guideline-based therapy, including β-blockers [BB]) was associated with a reduction of cardiovascular death and hospitalizations for HF, but BB were underutilized. Further studies are needed to test the efficacy of ivabradine in CAD patients with high HR and to shed light on the comparison between ivabradine and a more aggressive therapy with higher doses of BB in HF patients.
Collapse
Affiliation(s)
- Gian Marco Rosa
- University of Genoa, San Martino Hospital and National Institute for Cancer Research, Department of Cardiology , Genoa , Italy
| | | | | | | | | |
Collapse
|
13
|
Severi S, Fantini M, Charawi LA, DiFrancesco D. An updated computational model of rabbit sinoatrial action potential to investigate the mechanisms of heart rate modulation. J Physiol 2012; 590:4483-99. [PMID: 22711956 DOI: 10.1113/jphysiol.2012.229435] [Citation(s) in RCA: 69] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
The cellular basis of cardiac pacemaking is still debated. Reliable computational models of the sinoatrial node (SAN) action potential (AP) may help gain a deeper understanding of the phenomenon. Recently, novel models incorporating detailed Ca(2+)-handling dynamics have been proposed, but they fail to reproduce a number of experimental data, and more specifically effects of 'funny' (I(f)) current modifications. We therefore developed a SAN AP model, based on available experimental data, in an attempt to reproduce physiological and pharmacological heart rate modulation. Cell compartmentalization and intracellular Ca(2+)-handling mechanisms were formulated as in the Maltsev-Lakatta model, focusing on Ca(2+)-cycling processes. Membrane current equations were revised on the basis of published experimental data. Modifications of the formulation of currents/pumps/exchangers to simulate I(f) blockers, autonomic modulators and Ca(2+)-dependent mechanisms (ivabradine, caesium, acetylcholine, isoprenaline, BAPTA) were derived from experimental data. The model generates AP waveforms typical of rabbit SAN cells, whose parameters fall within the experimental ranges: 352 ms cycle length, 80 mV AP amplitude, -58 mV maximum diastolic potential (MDP), 108 ms APD(50), and 7.1 Vs(-1) maximum upstroke velocity. Rate modulation by I(f) -blocking drugs agrees with experimental findings: 20% and 22% caesium-induced (5mM) and ivabradine-induced (3 μM) rate reductions, respectively, due to changes in diastolic depolarization (DD) slope, with no changes in either MDP or take-off potential (TOP). The model consistently reproduces the effects of autonomic modulation: 20% rate decrease with 10 nM acetylcholine and 28%increase with 1 μM isoprenaline, again entirely due to increase in the DD slope,with no changes in either MDP or TOP. Model testing of BAPTA effects showed slowing of rate, -26%, without cessation of beating. Our up-to-date model describes satisfactorily experimental data concerning autonomic stimulation, funny-channel blockade and inhibition of the Ca(2+)-related system by BAPTA, making it a useful tool for further investigation. Simulation results suggest that a detailed description of the intracellular Ca(2+) fluxes is fully compatible with the observation that I(f) is a major component of pacemaking and rate modulation.
Collapse
Affiliation(s)
- Stefano Severi
- Biomedical Engineering Laboratory - DEIS, University of Bologna, Via Venezia 52, 47521 Cesena, Italy.
| | | | | | | |
Collapse
|
14
|
Scicchitano P, Carbonara S, Ricci G, Mandurino C, Locorotondo M, Bulzis G, Gesualdo M, Zito A, Carbonara R, Dentamaro I, Riccioni G, Ciccone MM. HCN channels and heart rate. Molecules 2012; 17:4225-35. [PMID: 22481543 PMCID: PMC6268830 DOI: 10.3390/molecules17044225] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2011] [Revised: 03/21/2012] [Accepted: 03/30/2012] [Indexed: 11/16/2022] Open
Abstract
Hyperpolarization and Cyclic Nucleotide (HCN) -gated channels represent the molecular correlates of the "funny" pacemaker current (I(f)), a current activated by hyperpolarization and considered able to influence the sinus node function in generating cardiac impulses. HCN channels are a family of six transmembrane domain, single pore-loop, hyperpolarization activated, non-selective cation channels. This channel family comprises four members: HCN1-4, but there is a general agreement to consider HCN4 as the main isoform able to control heart rate. This review aims to summarize advanced insights into the structure, function and cellular regulation of HCN channels in order to better understand the role of such channels in regulating heart rate and heart function in normal and pathological conditions. Therefore, we evaluated the possible therapeutic application of the selective HCN channels blockers in heart rate control.
Collapse
Affiliation(s)
- Pietro Scicchitano
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Santa Carbonara
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Gabriella Ricci
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Cosimo Mandurino
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Manuela Locorotondo
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Gabriella Bulzis
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Michele Gesualdo
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Annapaola Zito
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Rosa Carbonara
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Ilaria Dentamaro
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
| | - Graziano Riccioni
- Cardiology Unit, San Camillo De Lellis Hospital, Manfredonia (FG) 71043, Italy
| | - Marco Matteo Ciccone
- Section of Cardiovascular Diseases, Department of Emergency and Organ Transplantation, University of Bari, School of Medicine, Policlinico, Bari 70124, Italy
- Author to whom correspondence should be addressed; ; Tel.: +39-080-5478791; Fax: +39-080-5478796
| |
Collapse
|
15
|
Collins TP, Terrar DA. Ca(2+)-stimulated adenylyl cyclases regulate the L-type Ca(2+) current in guinea-pig atrial myocytes. J Physiol 2012; 590:1881-93. [PMID: 22351635 DOI: 10.1113/jphysiol.2011.227066] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Ca(2+)-stimulated adenylyl cyclases (ACs) have recently been shown to play important roles in pacemaking in the sino-atrial node. Here we present evidence that Ca(2+)-stimulated ACs are functionally active in guinea-pig atrial myocytes. Basal activity of an AC in isolated atrial myocytes was demonstrated by the observations that MDL 12,330A (10 μm), an AC inhibitor, reduced L-type Ca(2+) current (I(CaL)) amplitude, while inhibition of phosphodiesterases with IBMX (100 μm) increased I(CaL) amplitude. Buffering of cytosolic Ca(2+) by exposure of myocytes to BAPTA-AM (5 μm) reduced I(CaL) amplitude, as did inhibition of Ca(2+) release from the sarcoplasmic reticulum with ryanodine (2 μm) and thapsigargin (1 μm). [Ca(2+)]i-activated calmodulin kinase II (CaMKII) inhibition with KN-93 (1 μm) reduced I(CaL), but subsequent application of BAPTA-AM further reduced I(CaL). This effect of BAPTA-AM, in the presence of CaMKII inhibition, demonstrates that there is an additional Ca(2+)-modulated pathway (not dependent on CaMKII) that regulates I(CaL) in atrial myocytes. The effects of BAPTA could be reversed by forskolin (10 μm), a direct stimulator of all AC isoforms, which would restore cAMP levels. In the presence of BAPTA-AM, the actions of IBMX were reduced. In addition, inclusion of cAMP in the patch electrode in the whole-cell configuration prevented the effects of BAPTA. These effects are all consistent with a role for Ca(2+)-stimulated AC in the regulation of atrial myocyte I(CaL).
Collapse
Affiliation(s)
- Thomas P Collins
- National Heart and Lung Institute, Imperial College, Guy Scadding Building, Dovehouse Street, London SW3 6LY, UK.
| | | |
Collapse
|
16
|
Han SY, Bolter CP. The muscarinic-activated potassium channel always participates in vagal slowing of the guinea-pig sinoatrial pacemaker. Auton Neurosci 2011; 164:96-100. [PMID: 21684818 DOI: 10.1016/j.autneu.2011.05.009] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2011] [Revised: 05/23/2011] [Accepted: 05/26/2011] [Indexed: 10/18/2022]
Abstract
UNLABELLED Controversy persists regarding participation of the muscarinic-activated potassium current (c(KACh)) in small and moderate vagal bradycardia. We investigated this by (i) critical examination of earlier experimental data for mechanisms proposed to operate in modest vagal bradycardia (modulation of I(f) and inhibition of a junctional Na(+) current) and (ii) experiments performed on isolated vagally-innervated guinea-pig atria. In 8 superperfused preparations, 10-s trains of vagal stimulation (1 to 20Hz) produced a bradycardia that ranged from 1 to 80%. Hyperpolarisation of sinoatrial cells accompanied bradycardia in 65/67 observations (linear correlation between bradycardia and increase in maximum diastolic potential (mV)=0.076x%; R(2)=0.57; P<0.001). In bath-mounted preparations single supramaximal stimuli to the vagus immediately and briefly increased pacemaker cycle length in 7 of 18 preparations. This response was eliminated by 300nM tertiapin-Q. Trains of 10 single supramaximal vagal stimuli applied at 1-s intervals caused progressive increase in overall cycle length during the train; immediate and brief increases in cycle length occurred following some stimuli. Immediate brief responses and part of the slower response to the stimulus train were removed by 300nM tertiapin-Q. SUMMARY experimental data shows that small and modest vagal bradycardia is accompanied by hyperpolarisation of the pacemaker cell which is severely attenuated by tertiapin-Q. These observations support the idea that activation of I(KACh) occurs at all levels of vagal bradycardia. Contradictory conclusions from earlier studies may be attributed to the nature of experimental models and experimental design.
Collapse
Affiliation(s)
- Su Young Han
- Department of Physiology and the Centre for Neuroscience, University of Otago, Dunedin, New Zealand
| | | |
Collapse
|
17
|
Abstract
Abstract: Pacemaking is a basic physiological process, and the cellular mechanisms involved in this function have always attracted the keen attention of investigators. The "funny" (I(f)) current, originally described in sinoatrial node myocytes as an inward current activated on hyperpolarization to the diastolic range of voltages, has properties suitable for generating repetitive activity and for modulating spontaneous rate. The degree of activation of the funny current determines, at the end of an action potential, the steepness of phase 4 depolarization; hence, the frequency of action potential firing. Because I(f) is controlled by intracellular cAMP and is thus activated and inhibited by beta-adrenergic and muscarinic M2 receptor stimulation, respectively, it represents a basic physiological mechanism mediating autonomic regulation of heart rate. Given the complexity of the cellular processes involved in rhythmic activity, an exact quantification of the extent to which I(f) and other mechanisms contribute to pacemaking is still a debated issue; nonetheless, a wealth of information collected since the current was first described more than 30 years ago clearly agrees to identify I(f) as a major player in both generation of spontaneous activity and rate control. I(f)- dependent pacemaking has recently advanced from a basic, physiologically relevant concept, as originally described, to a practical concept that has several potentially useful clinical applications and can be valuable in therapeutically relevant conditions. Typically, given their exclusive role in pacemaking, f-channels are ideal targets of drugs aiming to pharmacological control of cardiac rate. Molecules able to bind specifically to and block f-channels can thus be used as pharmacological tools for heart rate reduction with little or no adverse cardiovascular side effects. Indeed a selective f-channel inhibitor, ivabradine, is today commercially available as a tool in the treatment of stable chronic angina. Also, several loss-of-function mutations of HCN4 (hyperpolarization-activated, cyclic-nucleotide gated 4), the major constitutive subunit of f-channels in pacemaker cells, are known today to cause rhythm disturbances, such as for example inherited sinus bradycardia. Finally, gene- or cell-based methods for in situ delivery of f-channels to silent or defective cardiac muscle represent novel approaches for the development of biological pacemakers eventually able to replace electronic devices.
Collapse
Affiliation(s)
- Dario DiFrancesco
- University of Milano, Department of Biomolecular Sciences and Biotechnology, The PaceLab, via Celoria 26, 20133 Milano, Italy.
| |
Collapse
|
18
|
Barbuti A, Galvez BG, Crespi A, Scavone A, Baruscotti M, Brioschi C, Cossu G, DiFrancesco D. Mesoangioblasts from ventricular vessels can differentiate in vitro into cardiac myocytes with sinoatrial-like properties. J Mol Cell Cardiol 2010; 48:415-23. [DOI: 10.1016/j.yjmcc.2009.10.006] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/17/2009] [Revised: 09/07/2009] [Accepted: 10/02/2009] [Indexed: 10/20/2022]
|
19
|
Heart function influenced by selective mid-cervical left vagus nerve stimulation in a human case study. Hypertens Res 2009; 32:1041-3. [DOI: 10.1038/hr.2009.140] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
|
20
|
Biel M, Wahl-Schott C, Michalakis S, Zong X. Hyperpolarization-activated cation channels: from genes to function. Physiol Rev 2009; 89:847-85. [PMID: 19584315 DOI: 10.1152/physrev.00029.2008] [Citation(s) in RCA: 719] [Impact Index Per Article: 47.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a small subfamily of proteins within the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises four members (HCN1-4) that are expressed in heart and nervous system. The current produced by HCN channels has been known as I(h) (or I(f) or I(q)). I(h) has also been designated as pacemaker current, because it plays a key role in controlling rhythmic activity of cardiac pacemaker cells and spontaneously firing neurons. Extensive studies over the last decade have provided convincing evidence that I(h) is also involved in a number of basic physiological processes that are not directly associated with rhythmicity. Examples for these non-pacemaking functions of I(h) are the determination of the resting membrane potential, dendritic integration, synaptic transmission, and learning. In this review we summarize recent insights into the structure, function, and cellular regulation of HCN channels. We also discuss in detail the different aspects of HCN channel physiology in the heart and nervous system. To this end, evidence on the role of individual HCN channel types arising from the analysis of HCN knockout mouse models is discussed. Finally, we provide an overview of the impact of HCN channels on the pathogenesis of several diseases and discuss recent attempts to establish HCN channels as drug targets.
Collapse
Affiliation(s)
- Martin Biel
- Center for Integrated Protein Science CIPS-M and Zentrum für Pharmaforschung, Department Pharmazie, Pharmakologie für Naturwissenschaften, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, Munich D-81377, Germany.
| | | | | | | |
Collapse
|
21
|
Bolter CP, Turner MJ. Tertiapin-Q removes a large and rapidly acting component of vagal slowing of the guinea-pig cardiac pacemaker. Auton Neurosci 2009; 150:76-81. [PMID: 19481505 DOI: 10.1016/j.autneu.2009.05.244] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2008] [Revised: 05/01/2009] [Accepted: 05/01/2009] [Indexed: 11/17/2022]
Abstract
The participation of acetylcholine-activated potassium current (I(K,ACh)) and hyperpolarization-activated pacemaker current (I(f)) in vagal bradycardia were examined using vagally-innervated preparations of guinea-pig atria. Preparations were maintained in Krebs-Henseleit solution (36 degrees C). Before treatment, trains of vagal stimuli (10 s at 2, 5 and 10 Hz) produced graded bradycardias displaying rapid onset and offset. Tertiapin-Q (300 nM), which blocks I(K,ACh), had no effect on baseline atrial rate. In tertiapin-Q, vagal bradycardia displayed a gradual onset and offset, with a peak response ~50% of that recorded in control conditions. Cumulative addition of 1 mM ZD7288 (blocker of I(f)) caused atrial rate to fall by ~60%, but had no further effect on the amplitude of the vagal bradycardia, while response onset and offset became slightly faster. From these observations, we argue that (i) vagal bradycardia was attributable primarily to activation of I(K,ACh), (ii) vagal modulation of I(f) had a minor influence on the rate of onset and offset of bradycardia, and (iii) removal of the influence of I(K,ACh) unmasked a slow response, of undetermined origin, to vagal stimulation. In a separate set of experiments we compared the effects of 1 mM Ba(2+) and 300 nM tertiapin-Q on vagal bradycardia. Ba(2+) reduced baseline atrial rate and the response to vagal stimulation. Subsequent cumulative addition of tertiapin-Q had no additional effect on baseline atrial rate, but caused further reduction in the amplitude of vagal bradycardia, suggesting that 1 mM Ba(2+) did not achieve a complete block of I(K,ACh) in this preparation.
Collapse
Affiliation(s)
- Chris P Bolter
- Department of Physiology and the Centre for Neuroscience, University of Otago, Dunedin, New Zealand.
| | | |
Collapse
|
22
|
Lakatta EG, DiFrancesco D. What keeps us ticking: a funny current, a calcium clock, or both? J Mol Cell Cardiol 2009; 47:157-70. [PMID: 19361514 DOI: 10.1016/j.yjmcc.2009.03.022] [Citation(s) in RCA: 205] [Impact Index Per Article: 13.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/05/2009] [Revised: 03/10/2009] [Accepted: 03/19/2009] [Indexed: 12/14/2022]
Affiliation(s)
- Edward G Lakatta
- Laboratory of Cardiovascular Science, National Institute on Aging, Intramural Research Program, NIH, 5600 Nathan Shock Drive, Baltimore, MD 21224-6825, USA.
| | | |
Collapse
|
23
|
Characterization of the Heart Rate-Lowering Action of Ivabradine, a Selective I f Current Inhibitor. Am J Ther 2008; 15:461-73. [PMID: 18806523 DOI: 10.1097/mjt.0b013e3181758855] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
|
24
|
Barbuti A, DiFrancesco D. Control of cardiac rate by "funny" channels in health and disease. Ann N Y Acad Sci 2008; 1123:213-23. [PMID: 18375593 DOI: 10.1196/annals.1420.024] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
Activation of the "funny" (pacemaker, I f) current during the diastolic depolarization phase of an action potential is the main mechanism underlying spontaneous, rhythmic activity of cardiac pacemaker cells. In the past three decades, a wealth of evidence elucidating the function of the funny current in the generation and modulation of cardiac pacemaker activity has been gathered. The slope of early diastolic depolarization, and thus the heart rate, is controlled precisely by the degree of I f activation during diastole. I f is also accurately and rapidly modulated by changes of the cytosolic concentration of the second messenger cAMP, operated by the autonomous nervous system through beta-adrenergic, mainly beta2, and in the opposite way by muscarinic receptor, stimulation. Recently, novel in vivo data, both in animal models and humans, have been collected that confirm the key role of I f in pacemaking. In particular, an inheritable point mutation in the cyclic nucleotide-binding domain of human HCN4, the main hyperpolarization-activated cyclic nucleotide (HCN) isoform contributing to native funny channels of the sinoatrial node, was shown to be associated with sinus bradycardia in a large family. Because of their properties, funny channels have long been a major target of classical pharmacological research and are now target of innovative gene/cell-based therapeutic approaches aimed to exploit their function in cardiac rate control.
Collapse
Affiliation(s)
- Andrea Barbuti
- Department of Biomolecular Sciences and Biotechnology, The PaceLab, University of Milan, Milan, Italy
| | | |
Collapse
|
25
|
Stamatiou K, Heretis I, Skoumbourdis E. Does ivabradine exhibit a role in the reduction of bladder overactivity? Int Urol Nephrol 2008; 40:333-4. [DOI: 10.1007/s11255-007-9329-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2007] [Accepted: 12/21/2007] [Indexed: 10/22/2022]
|
26
|
Cardiac pacemaker function of HCN4 channels in mice is confined to embryonic development and requires cyclic AMP. EMBO J 2008; 27:692-703. [PMID: 18219271 DOI: 10.1038/emboj.2008.3] [Citation(s) in RCA: 90] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2007] [Accepted: 12/21/2007] [Indexed: 11/08/2022] Open
Abstract
Important targets for cAMP signalling in the heart are hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels that underlie the depolarizing 'pacemaker' current, I(f). We studied the role of I(f) in mice, in which binding of cAMP to HCN4 channels was abolished by a single amino-acid exchange (R669Q). Homozygous HCN4(R669Q/R669Q) mice die during embryonic development. Prior to E12, homozygous and heterozygous embryos display reduced heart rates and show no or attenuated responses to catecholaminergic stimulation. Adult heterozygous mice display normal heart rates at rest and during exercise. However, following beta-adrenergic stimulation, hearts exhibit pauses and sino-atrial node block. Our results demonstrate that in the embryo, HCN4 is a true cardiac pacemaker and elevation of HCN4 channel activity by cAMP is essential for viability. In adult mice, an important function of HCN4 channels is to prevent sinus pauses during and after stress while their role as a pacemaker of the murine heart is put into question. Most importantly, our results indicate that HCN4 channels can fulfil their physiological function only when cAMP is bound.
Collapse
|
27
|
Rose RA, Giles WR. Natriuretic peptide C receptor signalling in the heart and vasculature. J Physiol 2007; 586:353-66. [PMID: 18006579 DOI: 10.1113/jphysiol.2007.144253] [Citation(s) in RCA: 155] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Natriuretic peptides (NPs), including atrial, brain and C-type natriuretic peptides (ANP, BNP and CNP), bind two classes of cell surface receptors: the guanylyl cyclase-linked A and B receptors (NPR-A and NPR-B) and the C receptor (NPR-C). The biological effects of NPs have been mainly attributed to changes in intracellular cGMP following their binding to NPR-A and NPR-B. NPR-C does not include a guanylyl cyclase domain. It has been denoted as a clearance receptor and is thought to bind and internalize NPs for ultimate degradation. However, a substantial body of biochemical work has demonstrated the ability of NPR-C to couple to inhibitory G proteins (Gi) and cause inhibition of adenylyl cyclase and activation of phospholipase-C. Recently, novel physiological effects of NPs, mediated specifically by NPR-C, have been discovered in the heart and vasculature. We have described the ability of CNP, acting via NPR-C, to selectively inhibit L-type calcium currents in atrial and ventricular myocytes, as well as in pacemaker cells (sinoatrial node myocytes). In contrast, our studies of the electrophysiological effects of CNP on cardiac fibroblasts demonstrated an NPR-C-Gi-phospholipase-C-dependent activation of a non-selective cation current mediated by transient receptor potential (TRP) channels. It is also known that CNP and BNP have important anti-proliferative effects in cardiac fibroblasts that appear to involve NPR-C. In the mammalian resistance vessels, including mesenteric and coronary arteries, CNP has been found to function as an NPR-C-dependent endothelium-derived hyperpolarizing factor that regulates local blood flow and systemic blood pressure by hyperpolarizing smooth muscle cells. In this review we highlight the role of NPR-C in mediating these NP effects in myocytes and fibroblasts from the heart as well as in vascular smooth muscle cells.
Collapse
Affiliation(s)
- Robert A Rose
- Departments of Physiology, Heart and Stroke/Richard Lewar Centre, University of Toronto and University Health Network, Toronto, Ontario, Canada M5S 3E2.
| | | |
Collapse
|
28
|
Bolter CP, English DJ. The effects of tertiapin-Q on responses of the sinoatrial pacemaker of the guinea-pig heart to vagal nerve stimulation and muscarinic agonists. Exp Physiol 2007; 93:53-63. [PMID: 17720744 DOI: 10.1113/expphysiol.2007.038901] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Using Langendorff preparations of the guinea-pig heart, we have examined the participation of the acetylcholine (ACh)-activated potassium channel, IK,ACh, in the bradycardia produced by electrical stimulation of the vagus (parasympathetic) nerve and muscarinic agonists (ACh and bethanecol, bolus i.a.). Hearts from young animals (160-250 g) were perfused with Krebs-Henseleit solution, and pacemaker frequency was determined from the P wave of an ECG. Tertiapin-Q was used to block IK,ACh. Vagal stimulation (10 s trains at 2, 5 and 10 Hz) produced graded reductions in atrial rate that were substantially attenuated, and to a similar extent, by 300 nm and 1 microm tertiapin-Q (to 0.42 +/- 0.12, mean +/- s.d., of the control values; P < 0.001). Acetylcholine (3 nmol) produced brief graded bradycardias that were also attenuated by tertiapin-Q (0.24 +/- 0.24; P = 0.006). Similar results were obtained when experiments were repeated in 2 mm Cs+ (to block the hyperpolarization-activated pacemaker current). Bethanecol (30, 50 and 70 nmol), a muscarinic agonist with no appreciable nicotinic activity, produced sustained bradycardias that were attenuated by 300 nm tertiapin-Q (0.36 +/- 0.21; P < 0.0001). The responses to vagal stimulation and ACh developed more slowly in tertiapin-Q, indicating that a rapidly acting mechanism had been blocked. Responses to vagal stimulation were faster in 2 mm Cs+. Together, these observations show that ACh released from parasympathetic nerve varicosities exerts a considerable part of its effect on the pacemaker by activating IK,ACh and acts in a manner not readily distinguishable from that of directly applied muscarinic agonists.
Collapse
Affiliation(s)
- Chris P Bolter
- Department of Physiology, School of Medical Sciences, University of Otago, PO Box 913, Dunedin 9054, New Zealand.
| | | |
Collapse
|
29
|
Kuba H. Cellular and molecular mechanisms of avian auditory coincidence detection. Neurosci Res 2007; 59:370-6. [PMID: 17884214 DOI: 10.1016/j.neures.2007.08.003] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2007] [Revised: 08/06/2007] [Accepted: 08/10/2007] [Indexed: 10/22/2022]
Abstract
Sound localization along the horizontal plane begins with comparing sound arrival times at the two ears in the brainstem coincidence detector neurons. Coincidence detectors are functionally as well as morphologically specialized depending on the frequency of sounds that they handle, and the expression of various channel molecules underlies these specializations. Some voltage-gated K(+) channels determine the acuity of coincidence detection, and are expressed most abundantly in the middle-frequency-coding neurons. Some hyperpolarization-activated channels are dominant in the high-frequency-coding neurons, and enable a delicate modulation of coincidence detection by noradrenalin. Axonal clustering of Na(+) channels is also frequency-dependent, and optimizes the coincidence detection. This article aims to provide an overview of recent findings in the cellular and molecular mechanisms of auditory coincidence detection in birds.
Collapse
Affiliation(s)
- Hiroshi Kuba
- Career-Path Promotion Unit For Young Life Scientists, Kyoto University Graduate School of Medicine, Kyoto, Japan.
| |
Collapse
|
30
|
Vassalle M. The vicissitudes of the pacemaker current I Kdd of cardiac purkinje fibers. J Biomed Sci 2007; 14:699-716. [PMID: 17564816 DOI: 10.1007/s11373-007-9182-2] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2007] [Accepted: 05/10/2007] [Indexed: 01/23/2023] Open
Abstract
The mechanisms underlying the pacemaker current in cardiac tissues is not agreed upon. The pacemaker potential in Purkinje fibers has been attributed to the decay of the potassium current I (Kdd). An alternative proposal is that the hyperpolarization-activated current I (f) underlies the pacemaker potential in all cardiac pacemakers. The aim of this review is to retrace the experimental development related to the pacemaker mechanism in Purkinje fibers with reference to findings about the pacemaker mechanism in the SAN as warranted. Experimental data and their interpretation are critically reviewed. Major findings were attributed to K(+) depletion in narrow extracellular spaces which would result in a time dependent decay of the inward rectifier current I (K1). In turn, this decay would be responsible for a "fake" reversal of the pacemaker current. In order to avoid such a postulated depletion, Ba(2+) was used to block the decay of I (K1). In the presence of Ba(2+) the time-dependent current no longer reversed and instead increased with time and more so at potentials as negative as -120 mV. In this regard, the distinct possibility needs to be considered that Ba(2+) had blocked I (Kdd) (and not only I (K1)). That indeed this was the case was demonstrated by studying single Purkinje cells in the absence and in the presence of Ba(2+). In the absence of Ba(2+), I (Kdd) was present in the pacemaker potential range and reversed at E (K). In the presence of Ba(2+), I (Kdd) was blocked and I (f) appeared at potentials negative to the pacemaker range. The pacemaker potential behaves in a manner consistent with the underlying I (Kdd) but not with I (f). The fact that I (f) is activated on hyperpolarization at potential negative to the pacemaker range makes it suitable as a safety factor to prevent the inhibitory action of more negative potentials on pacemaker discharge. It is concluded that the large body of evidence reviewed proves the pacemaker role of I (Kdd) (but not of I (f)) in Purkinje fibers.
Collapse
Affiliation(s)
- Mario Vassalle
- Department of Physiology and Pharmacology, Box 31 State University of New York, Downstate Medical Center, 450 Clarkson Avenue, Brooklyn, NY 11203, USA.
| |
Collapse
|
31
|
Mattick P, Parrington J, Odia E, Simpson A, Collins T, Terrar D. Ca2+-stimulated adenylyl cyclase isoform AC1 is preferentially expressed in guinea-pig sino-atrial node cells and modulates the I(f) pacemaker current. J Physiol 2007; 582:1195-203. [PMID: 17540702 PMCID: PMC2075242 DOI: 10.1113/jphysiol.2007.133439] [Citation(s) in RCA: 98] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
Ca(2+)-stimulated adenylyl cyclases (AC) are known to play important roles in neurons but have not previously been reported in the heart. Here we present the first evidence for selective expression of Ca(2+)-stimulated AC in the sino-atrial node (SAN) but not in ventricular muscle of the guinea-pig heart. The AC1 isoform of Ca(2+)-stimulated AC was shown to be present in SAN, both as mRNA using RT-PCR and as protein using immuno-blotting with a specific antibody. Confocal immuno-fluorescence studies detected membrane localization of AC1 in SAN cells, but no AC1 in ventricular muscle. Ca(2+)-stimulated AC8 may also be present in SAN. The functional importance of AC activity was investigated by monitoring activation of I(f) (gated by hyperpolarization and regulated by cAMP, which shifts activation to more depolarized voltages). Basal activity of AC in isolated SAN myocytes was demonstrated by the observations that an inhibitor of AC activity (MDL 12330A, 10 microm) shifted activation in the hyperpolarizing direction, while inhibition of phosphodiesterases (IBMX, 100 microm) shifted I(f) activation in the depolarizing direction. Buffering cytosolic Ca(2+) with the Ca(2+) chelator BAPTA (by exposure to BAPTA-AM) shifted activation of I(f) in the hyperpolarizing direction, and under these conditions the AC inhibitor MDL had little or no further effect. The actions of BAPTA were overcome by exposure to forskolin (10 microm), a direct stimulator of all AC isoforms, to restore cAMP levels. These effects are consistent with the functional importance of Ca(2+)-stimulated AC, which is expected to be fundamental to initiation and regulation of the heartbeat.
Collapse
Affiliation(s)
- Paul Mattick
- University Department of Pharmacology, Mansfield Road, Oxford, OX1 3QT, UK
| | | | | | | | | | | |
Collapse
|
32
|
Fedorov VV, Hucker WJ, Dobrzynski H, Rosenshtraukh LV, Efimov IR. Postganglionic nerve stimulation induces temporal inhibition of excitability in rabbit sinoatrial node. Am J Physiol Heart Circ Physiol 2006; 291:H612-23. [PMID: 16565321 DOI: 10.1152/ajpheart.00022.2006] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Vagal stimulation results in complex changes of pacemaker excitability in the sinoatrial node (SAN). To investigate the vagal effects in the rabbit SAN, we used optical mapping, which is the only technology that allows resolving simultaneous changes in the activation pattern and action potentials morphologies. With the use of immunolabeling, we identified the SAN as a neurofilament 160-positive but connexin 43-negative region ( n = 5). Normal excitation originated in the SAN center with a cycle length (CL) of 405 ± 14 ms ( n = 14), spread anisotropically along the crista terminalis (CT), and failed to conduct toward the septum. Postganglionic nerve stimulation (PNS, 400–800 ms) reduced CL by 74 ± 7% transiently and shifted the leading pacemaker inferiorly (78%) or superiorly (22%) from the SAN center by 2–10 mm. In the intercaval region between the SAN center and the septal block zone, PNS produced an 8 ± 1-mm2 region of transient hyperpolarization and inexcitability. The first spontaneous or paced excitation following PNS could not enter this region for 500–1,500 ms. Immunolabeling revealed that the PNS-induced inexcitable region is located between the SAN center and the block zone and has a 2.5-fold higher density of choline acetyltransferase than CT but is threefold lower than the SAN center. The fact that the inexcitability region does not coincide with the most innervated area indicates that the properties of the myocytes themselves, as well as intercellular coupling, must play a role in the inexcitability induction. Optically mapping revealed that PNS resulted in transient loss of pacemaker cell excitability and unidirectional entrance conduction block in the periphery of SAN.
Collapse
Affiliation(s)
- Vadim V Fedorov
- Department of Biomedical Engineering, Washington Univ., Campus Box 1097, One Brookings Drive, St. Louis, MO 63130, USA
| | | | | | | | | |
Collapse
|
33
|
Yamada R, Kuba H, Ishii TM, Ohmori H. Hyperpolarization-activated cyclic nucleotide-gated cation channels regulate auditory coincidence detection in nucleus laminaris of the chick. J Neurosci 2006; 25:8867-77. [PMID: 16192376 PMCID: PMC6725590 DOI: 10.1523/jneurosci.2541-05.2005] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Coincidence detection of bilateral acoustic signals in nucleus laminaris (NL) is the first step in azimuthal sound source localization in birds. Here, we demonstrate graded expression of hyperpolarization-activated cyclic nucleotide-gated (HCN) cation channels along the tonotopic axis of NL and its role in the regulation of coincidence detection. Expression of HCN1 and HCN2, but not HCN3 or HCN4, was detected in NL. Based on measurement of both subtype mRNA and protein, HCN1 varied along the tonotopic axis and was minimal in high-characteristic frequency (CF) neurons. In contrast, HCN2 was evenly distributed. The resting conductance was larger and the steady-state activation curve of Ih was more positive in neurons of middle to low CF than those of high CF, consistent with the predominance of HCN1 channels in these neurons. Application of 8-Br-cAMP or noradrenaline generated a depolarizing shift of the Ih voltage activation curve. This shift was larger in neurons of high CF than in those of middle CF. The shift in the activation voltage of Ih depolarized the resting membrane, accelerated the EPSP time course, and significantly improved the coincidence detection in neurons of high CF, suggesting that Ih may improve the localization of sound sources.
Collapse
Affiliation(s)
- Rei Yamada
- Department of Physiology, Faculty of Medicine, Kyoto University, Kyoto 606-8501, Japan
| | | | | | | |
Collapse
|
34
|
Abstract
Heart rate, a major determinant of angina in coronary disease, is also an important predictor of cardiovascular mortality. Lowering heart rate is therefore one of the most important therapeutic approaches in the treatment of stable angina pectoris. To date, beta-blockers and some calcium-channel antagonists reduce heart rate, but their use may be limited by adverse reactions or contraindications. Heart rate is determined by spontaneous electrical pacemaker activity in the sinoatrial node controlled by the I(f) current. Ivabradine is the first specific heart rate-lowering agent that has completed clinical development for stable angina pectoris. It is selective for the I(f) current, lowering heart rate at concentrations that do not affect other cardiac ionic currents. Specific heart-rate lowering with ivabradine reduces myocardial oxygen demand, simultaneously improving oxygen supply. Ivabradine has no negative inotropic or lusitropic effects, preserving ventricular contractility, and does not change any major electrophysiological parameters unrelated to heart rate. Randomised clinical studies in patients with stable angina show that ivabradine effectively reduces heart rate, improves exercise capacity and reduces the number of angina attacks. It has superior anti-anginal and anti-ischaemic activity to placebo and is non-inferior to atenolol and amlodipine. Ivabradine therefore offers a valuable approach to lowering heart rate exclusively and provides an attractive alternative to conventional treatment for a wide range of patients with confirmed stable angina.
Collapse
Affiliation(s)
- S Sulfi
- Department Cardiology, London Chest Hospital, London, UK
| | | |
Collapse
|
35
|
Milanesi R, Baruscotti M, Gnecchi-Ruscone T, DiFrancesco D. Familial sinus bradycardia associated with a mutation in the cardiac pacemaker channel. N Engl J Med 2006; 354:151-7. [PMID: 16407510 DOI: 10.1056/nejmoa052475] [Citation(s) in RCA: 281] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
We found that sinus bradycardia in members of a large family was associated with a mutation in the gene coding for the pacemaker HCN4 ion channel. Pacemaker channels of the sinoatrial node generate spontaneous activity and mediate cyclic AMP (cAMP)-dependent autonomic modulation of the heart rate. The mutation associated with bradycardia is located near the cAMP-binding site; functional analysis found that mutant channels respond normally to cAMP but are activated at more negative voltages than are wild-type channels. These changes, which mimic those of mild vagal stimulation, slow the heart rate by decreasing the inward diastolic current. Thus, diminished function of pacemaker channels is linked to familial bradycardia.
Collapse
Affiliation(s)
- Raffaella Milanesi
- Department of Biomolecular Sciences and Biotechnology, Laboratory of Molecular Physiology and Neurobiology, University of Milan, Milan, Italy
| | | | | | | |
Collapse
|
36
|
Borer JS. Drug insight: If inhibitors as specific heart-rate-reducing agents. ACTA ACUST UNITED AC 2006; 1:103-9. [PMID: 16265314 DOI: 10.1038/ncpcardio0052] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2004] [Accepted: 10/20/2004] [Indexed: 11/09/2022]
Abstract
Heart rate is determined primarily by spontaneously repeating net inward current carried by sodium ions and potassium ions through hyperpolarization-activated cyclic-nucleotide-gated channels. Within the heart, these channels are found most abundantly in sinoatrial cardiomyocytes. The channels open in response to membrane hyperpolarization, modulated by local cAMP concentrations. They permit activation of the I(f) current, which can be blocked specifically by molecules characterized by linked benzazepinone and benzocyclobutane rings, and which are devoid of effects on cardiac conduction, inotropy or peripheral vascular tone. The resulting heart-rate reduction has been effective in angina prevention in clinical trials involving 4,000 patients, using the prototype I(f) inhibitor, ivabradine. No serious adverse events have been attributed to the treatment; the most prominent side-effect is dose-related, always reversible and often transient visual symptoms that seldom result in voluntary drug discontinuation.
Collapse
Affiliation(s)
- Jeffrey S Borer
- Division of Cardiovascular Pathophysiology, Howard Gilman Institute for Valvular Heart Diseases, Weill Medical College of Cornell University, New York-Presbyterian Hospital Weill Cornell Center, New York, NY 10021, USA.
| |
Collapse
|
37
|
Cohen IS, Robinson RB. Pacemaker current and automatic rhythms: toward a molecular understanding. Handb Exp Pharmacol 2006:41-71. [PMID: 16610340 DOI: 10.1007/3-540-29715-4_2] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
The ionic basis of automaticity in the sinoatrial node and His-Purkinje system, the primary and secondary cardiac pacemaking regions, is discussed. Consideration is given to potential targets for pharmacologic or genetic therapies of rhythm disorders. An ideal target would be an ion channel that functions only during diastole, so that action potential repolarization is not affected, and one that exhibits regional differences in expression and/or function so that the primary and secondary pacemakers can be selectively targeted. The so-called pacemaker current, If, generated by the HCN gene family, best fits these criteria. The biophysical and molecular characteristics of this current are reviewed, and progress to date in developing selective pharmacologic agents targeting If and in using gene and cell-based therapies to modulate the current are reviewed.
Collapse
Affiliation(s)
- I S Cohen
- Department of Physiology and Biophysics, Stony Brook University, Room 150 Basic Science Tower, Stony Brook, NY 11794-8661, USA
| | | |
Collapse
|
38
|
Toyoda F, Ding WG, Matsuura H. Responses of the sustained inward current to autonomic agonists in guinea-pig sino-atrial node pacemaker cells. Br J Pharmacol 2005; 144:660-8. [PMID: 15678089 PMCID: PMC1576045 DOI: 10.1038/sj.bjp.0706101] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
1. The present study was undertaken to examine the responses of the sustained inward current (I(st)) to beta-adrenergic and muscarinic agonists in guinea-pig sino-atrial (SA) node cells using the whole-cell patch-clamp technique. I(st) was detected as the nicardipine (1 microM)-sensitive inward current at potentials between approximately -80 and +20 mV in the presence of low concentration (0.1 mM) of extracellular Ca2+, where the L-type Ca2+ current (I(Ca,L)) was practically abolished. 2. Beta-adrenergic agonist isoprenaline (ISO) in nanomolar concentrations not only increased the amplitude of I(st) but also shifted the membrane potential producing the peak amplitude (Vpeak) to a negative direction by approximately 15 mV without appreciably affecting potential range for the current activation. The stimulatory effect of ISO was concentration-dependent with an EC50 of 2.26 nM and the maximal effect (96.4+/-22.9% increase, n=6) was obtained at 100 nM ISO, when evaluated by the responses at -50 mV. 3. Bath application of acetylcholine (ACh) significantly inhibited I(st), which had been maximally augmented by 100 nM ISO; this inhibitory effect of ACh was concentration-dependent with an IC50 of 133.9 nM. High concentration (1000 nM) of ACh depressed basal I(st) by 10.5+/-2.0% (n=3). 4. In action potential clamp experiments, I(st) was also detected under control conditions and was markedly potentiated by exposure to ISO. 5. These results strongly suggest that I(st) not only contributes to the spontaneous action potentials of mammalian SA node cells but also plays a substantial role in mediating autonomic regulation of SA node pacemaker activity.
Collapse
Affiliation(s)
- Futoshi Toyoda
- Department of Physiology, Shiga University of Medical Science, Seta-tsukinowacho, Otsu, Shiga 520-2192, Japan.
| | | | | |
Collapse
|
39
|
Borer JS. Heart rate slowing by If inhibition: therapeutic utility from clinical trials. Eur Heart J Suppl 2005. [DOI: 10.1093/eurheartj/sui050] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
|
40
|
Baruscotti M, Bucchi A, Difrancesco D. Physiology and pharmacology of the cardiac pacemaker (“funny”) current. Pharmacol Ther 2005; 107:59-79. [PMID: 15963351 DOI: 10.1016/j.pharmthera.2005.01.005] [Citation(s) in RCA: 245] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2005] [Accepted: 01/26/2005] [Indexed: 12/19/2022]
Abstract
First described over a quarter of a century ago, the cardiac pacemaker "funny" (I(f)) current has been extensively characterized since, and its role in cardiac pacemaking has been thoroughly demonstrated. A similar current, termed I(h), was later described in different types of neurons, where it has a variety of functions and contributes to the control of cell excitability and plasticity. I(f) is an inward current activated by both voltage hyperpolarization and intracellular cAMP. In the heart, as well as generating spontaneous activity, f-channels mediate autonomic-dependent modulation of heart rate: beta-adrenergic stimulation accelerates, and vagal stimulation slows, cardiac rate by increasing and decreasing, respectively, the intracellular cAMP concentration and, consequently, the f-channel degree of activation. Four isoforms of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels have been cloned more recently and shown to be the molecular correlates of native f-channels in the heart and h-channels in the brain. Individual HCN isoforms have kinetic and modulatory properties which differ quantitatively. A comparison of their biophysical properties with those of native pacemaker channels provides insight into the molecular basis of the pacemaker current properties and, together with immunolabelling and other detection techniques, gives information on the pattern of HCN isoform distribution in different tissues. Because of their relevance to cardiac pacemaker activity, f-channels are a natural target of drugs aimed at the pharmacological control of heart rate. Several agents developed for their ability to selectively reduce heart rate act by a specific inhibition of f-channel function; these substances have a potential for the treatment of diseases such as angina and heart failure. In the near future, devices based on the delivery of f-channels in situ, or of a cellular source of f-channels (biological pacemakers), will likely be developed for use in therapies for diseases of heart rhythm with the aim of replacing electronic pacemakers.
Collapse
Affiliation(s)
- Mirko Baruscotti
- Laboratory of Molecular Physiology and Neurobiology, Department of Biomolecular Sciences and Biotechnology, University of Milano, via Celoria 26, 20133 Milan, Italy
| | | | | |
Collapse
|
41
|
Abstract
One of the most intriguing network-level inferences made on the basis of in vitro and modelling data regarding the role of Ih current was that they participate in rhythmogenesis in different parts of the brain. The nature of Ih contribution to various neuronal oscillations is far from uniform however, and the proper evaluation of the role of Ih in each particular structure requires in situ investigations in the intact brain. In this study we tested the effect of Ih blockade in the medial septum on hippocampal theta rhythm in anaesthetized and freely behaving rats. We could not confirm the recent report of elimination of theta by septal injection of ZD7288 [C. Xu et al. (2004) Eur. J. Neurosci., 19, 2299-2309]; the observed effects were more subtle and more specific. We found that Ih blockade in the medial septum substantially decreased the frequency of hippocampal oscillations without changing the context in which theta occurred, i.e. specific behaviours in freely moving rats and spontaneous switching and brainstem stimulation under anaesthesia. Septal injection of ZD7288 eliminated atropine-resistant theta elicited by high intensity electrical stimulation of the reticular formation in anaesthetized rats but was ineffective in combination with the muscarinic agonist, carbachol. Thus, functional Ih was necessary for the septum to generate or transmit high frequency theta rhythm elicited by strong ascending activation, whereas low frequency theta persisted after Ih blockade. These results suggest that Ih plays a specific role in septal theta generation by promoting fast oscillations during exploratory behaviour and rapid eye movement sleep.
Collapse
Affiliation(s)
- Bernat Kocsis
- Laboratory of Neurophysiology, Departments of Psychiatry at Massachusetts Mental Health Center and Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave, BT-551, Boston, MA 02215, USA.
| | | |
Collapse
|
42
|
Wang Z, Shi H, Wang H. Functional M3 muscarinic acetylcholine receptors in mammalian hearts. Br J Pharmacol 2004; 142:395-408. [PMID: 15148264 PMCID: PMC1574958 DOI: 10.1038/sj.bjp.0705787] [Citation(s) in RCA: 86] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2004] [Revised: 03/08/2004] [Accepted: 03/10/2004] [Indexed: 11/08/2022] Open
Abstract
In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.
Collapse
Affiliation(s)
- Zhiguo Wang
- Research Center, Montreal Heart Institute, University of Montreal, Montreal, Quebec, Canada.
| | | | | |
Collapse
|
43
|
|
44
|
Bucchi A, Baruscotti M, DiFrancesco D. Current-dependent block of rabbit sino-atrial node I(f) channels by ivabradine. J Gen Physiol 2002; 120:1-13. [PMID: 12084770 PMCID: PMC2238187 DOI: 10.1085/jgp.20028593] [Citation(s) in RCA: 189] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2002] [Revised: 04/29/2002] [Accepted: 05/06/2002] [Indexed: 11/30/2022] Open
Abstract
"Funny" (f-) channels have a key role in generation of spontaneous activity of pacemaker cells and mediate autonomic control of cardiac rate; f-channels and the related neuronal h-channels are composed of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel subunits. We have investigated the block of f-channels of rabbit cardiac sino-atrial node cells by ivabradine, a novel heart rate-reducing agent. Ivabradine is an open-channel blocker; however, block is exerted preferentially when channels deactivate on depolarization, and is relieved by long hyperpolarizing steps. These features give rise to use-dependent behavior. In this, the action of ivabradine on f-channels is similar to that reported of other rate-reducing agents such as UL-FS49 and ZD7288. However, other features of ivabradine-induced block are peculiar and do not comply with the hypothesis that the voltage-dependence of block is entirely attributable to either the sensitivity of ivabradine-charged molecules to the electrical field in the channel pore, or to differential affinity to different channel states, as has been proposed for UL-FS49 (DiFrancesco, D. 1994. Pflugers Arch. 427:64-70) and ZD7288 (Shin, S.K., B.S. Rotheberg, and G. Yellen. 2001. J. Gen. Physiol. 117:91-101), respectively. Experiments where current flows through channels is modified without changing membrane voltage reveal that the ivabradine block depends on the current driving force, rather than voltage alone, a feature typical of block induced in inwardly rectifying K(+) channels by intracellular cations. Bound drug molecules do not detach from the binding site in the absence of inward current through channels, even if channels are open and the drug is therefore not "trapped" by closed gates. Our data suggest that permeation through f-channel pores occurs according to a multiion, single-file mechanism, and that block/unblock by ivabradine is coupled to ionic flow. The use-dependence resulting from specific features of I(f) block by ivabradine amplifies its rate-reducing ability at high spontaneous rates and may be useful to clinical applications.
Collapse
Affiliation(s)
- Annalisa Bucchi
- Department of General Physiology and Biochemistry, Laboratory of Molecular Physiology and Neurobiology, and INFM-Unità Milano Università, 20133 Milano, Italy
| | | | | |
Collapse
|
45
|
Bolter CP, Wallace DJ, Hirst GD. Failure of Ba2+ and Cs+ to block the effects of vagal nerve stimulation in sinoatrial node cells of the guinea-pig heart. Auton Neurosci 2001; 94:93-101. [PMID: 11775712 DOI: 10.1016/s1566-0702(01)00355-1] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The aim of the study was to evaluate which ionic currents are modified in the sinoatrial node of guinea pigs when the vagus is stimulated. Responses of isolated atrial preparations to bilateral vagus nerve stimulation were examined. In bath-mounted preparations, 10-s trains of vagal stimulation (1-50 Hz) slowed the rate at which atrial contractions occurred. After the trains of stimuli, the force generated by each contraction was reduced. Both vago-inhibitory responses persisted in the presence of caesium (2 mM) and barium ions (1 mM). Vagal stimulation evoked a similar bradycardia in superperfused preparations in which intracellular recordings were made from pacemaker cells in the sinoatrial node. When pacemaking activity was abolished by adding the organic calcium channel antagonist nifedipine (1 microM) to the perfusate, vagal stimulation generated an inhibitory junction potential (IJP). Both the bradycardia and the amplitude of the inhibitory junction potential increased as the frequency of vagal stimulation was increased. The ability of vagal stimulation to produce inhibitory junction potentials was unaffected by the addition of caesium and barium ions to the perfusate. These observations suggest that the negative chronotropic and inotropic responses to vagal stimulation only minimally involve a muscarinically activated potassium current (I(KACh)) or changes in the hyperpolarization-activated pacemaker current Ih.
Collapse
Affiliation(s)
- C P Bolter
- Department of Physiology, University of Otago, Dunedin, New Zealand
| | | | | |
Collapse
|
46
|
Ono K, Masumiya H, Sakamoto A, Christé G, Shijuku T, Tanaka H, Shigenobu K, Ozaki Y. Electrophysiological analysis of the negative chronotropic effect of endothelin-1 in rabbit sinoatrial node cells. J Physiol 2001; 537:467-88. [PMID: 11731579 PMCID: PMC2278974 DOI: 10.1111/j.1469-7793.2001.00467.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
1. Electrophysiological effects of endothelin-1 (ET-1) were studied in rabbit sinoatrial node (SAN) using conventional microelectrode and whole-cell voltage and current recordings. 2. In rabbit SAN, RT-PCR detected ET(A) endothelin receptor mRNA. ET-1 (100 nM) increased the cycle length of action potentials (APs) from 305 +/- 15 to 388 +/- 25 ms; this effect was antagonised by the ET(A) receptor-selective antagonist BQ-123 (1 microM). ET-1 increased AP duration (APD50) by 22%, depolarised the maximum diastolic potential (MDP) from -59 +/- 1 to -53 +/- 2 mV, shifted the take-off potential by +5 mV and decreased the pacemaker potential (PMP) slope by 15%. Under exactly the same experimental conditions, ET-1 caused a positive chronotropic effect in guinea-pig SAN with a decrease of 13% in APD50, a shift of -4 mV in the take-off potential and an increase of 8% in the PMP slope. 3. Rabbit SAN exhibited two major cell types, distinguished both by their appearances and by their electrophysiological responses to ET-1. Whereas the spontaneous pacing rate and the PMP slope were similarly decreased by ET-1 (10 nM) in both cell types, ET-1 depolarised MDP from -67 +/- 1 to -62 +/- 4 mV in spindle-shaped cells but hyperpolarised it from -73 +/- 1 to -81 +/- 3 mV in rod-shaped cells. ET-1 decreased APD50 by 8 and 52% and shifted the take-off potential by +5 and -9 mV in spindle- and rod-shaped cells, respectively. 4. ET-1 decreased the high-threshold calcium current (I(CaL)) by about 50% in both cell types, without affecting its voltage dependence, and decreased the delayed rectifier K+ current (I(K)) with significant shifts (of +4.7 and +14.0 mV in spindle- and rod-shaped cells, respectively) in its voltage dependence. It was exclusively in rod-shaped cells that ET-1 activated a sizeable amount of time-independent inward-rectifying current. 5. The hyperpolarisation-activated current (I(f)), observed exclusively in spindle-shaped cells, was significantly increased by ET-1 at membrane potentials between -74.7 and -84.7 mV whereas it was significantly decreased at more negative potentials. ET-1 significantly decreased the slope of the current-voltage (I-V) relation of the I(f) tail without changing its half-maximum voltage. 6. The overall negative chronotropic influence of ET-1 on the whole rabbit SAN is interpreted as resulting from the integration of its different actions on spindle- and rod-shaped SAN cells through electrotonic interaction.
Collapse
Affiliation(s)
- K Ono
- Division of Chemical Pharmacology and Phytochemistry, National Institute of Health Sciences, 1-18-1 Kami-Yohga, Setagaya-ku, Tokyo 158-8501, Japan.
| | | | | | | | | | | | | | | |
Collapse
|
47
|
Abstract
In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.
Collapse
Affiliation(s)
- S Dhein
- Institute of Pharmacology, University of Halle-Wittenberg, Germany.
| | | | | |
Collapse
|
48
|
Cuttle MF, Rusznák Z, Wong AY, Owens S, Forsythe ID. Modulation of a presynaptic hyperpolarization-activated cationic current (I(h)) at an excitatory synaptic terminal in the rat auditory brainstem. J Physiol 2001; 534:733-44. [PMID: 11483704 PMCID: PMC2278738 DOI: 10.1111/j.1469-7793.2001.00733.x] [Citation(s) in RCA: 92] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
Abstract
1. A hyperpolarization-activated non-specific cation current, I(h), was examined in bushy cell bodies and their giant presynaptic terminals (calyx of Held). Whole-cell patch clamp recordings were made using an in vitro brain slice preparation of the cochlear nucleus and the superior olivary complex. The aim was to characterise I(h) in identified cell bodies and synaptic terminals, to examine modulation by presynaptic cAMP and to test for modulatory effects of I(h) activation on synaptic transmission. 2. Presynaptic I(h) was activated by hyperpolarizing voltage-steps, with half-activation (V(1/2)) at -94 mV. Activation time constants were voltage dependent, showing an e-fold acceleration for hyperpolarizations of -32 mV (time constant of 78 ms at -130 mV). The reversal potential of I(h) was -29 mV. It was blocked by external perfusion of 1 mM CsCl but was unaffected by BaCl(2). 3. Application of internal cAMP shifted the activation curve to more positive potentials, giving a V(1/2) of -74 mV; hence around half of the current was activated at resting membrane potentials. This shift in half-activation was mimicked by external perfusion of a membrane-permeant analogue, 8-bromo-cAMP. 4. The bushy cell body I(h) showed similar properties to those of the synaptic terminal; V(1/2) was -94 mV and the reversal potential was -33 mV. Somatic I(h) was blocked by CsCl (1 mM) and was partially sensitive to BaCl(2). Somatic I(h) current density increased with postnatal age from 5 to 16 days old, suggesting that I(h) is functionally relevant during maturation of the auditory pathway. 5. The function of I(h) in regulating presynaptic excitability is subtle. I(h) had little influence on EPSC amplitude at the calyx of Held, but may be associated with propagation of the action potential at branch points. Presynaptic I(h) shares properties with both HCN1 and HCN2 recombinant channel subunits, in that it gates relatively rapidly and is modulated by internal cAMP.
Collapse
Affiliation(s)
- M F Cuttle
- Ion Channel Group, Department of Cell Physiology and Pharmacology, University of Leicester, PO Box 138, Leicester LE1 9HN, UK
| | | | | | | | | |
Collapse
|
49
|
Abstract
It has been proposed that cholinergic agonists inhibit the sinoatrial node discharge by shifting the activation range of the hyperpolarization-activated inward current If to more negative values or by increasing potassium conductance. In the former instance, cesium will potentiate the cholinergic inhibition by blocking any residual If; in the latter instance, Cs+ and Ba2+ will antagonize the inhibitory action by blocking K+ channels. The changes in discharge induced by high and low concentrations of carbachol were studied using an electrophysiologic technique in isolated guinea pig sinoatrial node perfused in vitro in the absence and presence of different concentrations of Cs+ and Ba2+. In Tyrode solution, high carbachol concentrations (0.5-2 microM) slowed the sinoatrial node by hyperpolarizing the membrane and by reducing the amplitude of diastolic depolarization; and stopped the sinoatrial node by preventing the attainment of threshold potential. Adding Cs+ (10 mM) to carbachol increased the rate in slowly discharging sinoatrial node and induced spontaneous discharge in quiescent sinoatrial node. In high [K+]o (approximately 12 mM), carbachol slowed or stopped the slow responses and adding Cs+ accelerated or induced discharge. Both in Tyode and in high [K+]o, in the presence of Cs+, carbachol did stop the sinoatrial node. In the presence of carbachol, Ba2+ (0.1 mM) accelerated or induced discharge, as Cs+ did. Atropine (1 microM) prevented both the slowing or suppression by carbachol and the acceleration of sinoatrial node by Cs+ in the presence of carbachol. Low carbachol concentrations (0.05-0.1 microM) decreased the rate to a similar extent in the absence and the presence of a low concentration of Cs+ (2 mM, which blocks If but not K+ channels), but markedly less in the presence of 0.5-0.75 mM Ba2+ (which block K+ channels but not If). We conclude that cholinergic agonists slow or stop the sinoatrial node by a shifting the membrane potential toward the more negative subsidiary pacemaker range and away from the threshold. The results with Cs+ and Ba2+ indicate that both high and low concentrations of carbachol decrease sinoatrial node discharge by activating the I(K,ACh) channels rather than by decreasing If.
Collapse
Affiliation(s)
- M Vassalle
- Department of Physiology and Pharmacology, State University of New York, Brooklyn 11203, USA.
| | | |
Collapse
|
50
|
Nahavandi A, Dehpour AR, Mani AR, Homayounfar H, Abdoli A, Abdolhoseini MR. The role of nitric oxide in bradycardia of rats with obstructive cholestasis. Eur J Pharmacol 2001; 411:135-141. [PMID: 11137868 DOI: 10.1016/s0014-2999(00)00773-1] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Nitric oxide (NO) has an important role in controlling heart rate and contributes to the cholinergic antagonism of the positive chronotropic response to adrenergic stimulation. Based on evidence of NO overproduction in cholestasis and also on the existence of bradycardia in cholestatic subjects, this study aimed to evaluate the chronotropic effect of epinephrine in isolated atria of cholestatic rats and determine whether alterations in epinephrine-induced chronotropic responses of cholestatic rats are corrected after systemic inhibition of NO synthase (NOS) with N(G)-nitro-L-arginine (L-NNA). Male Sprague-Dawley rats were used. Cholestasis was induced by surgical ligation of the bile duct under general anesthesia and sham-operated animals were considered as control. The animals were divided into three groups, which received either L-arginine (200 mg/kg/day), L-NNA (10 mg/kg/day) or saline. One week after the operation, a lead II ECG was recorded from the animals, then spontaneously beating atria were isolated and chronotropic responses to epinephrine were evaluated in a standard oxygenated organ bath. The results showed that plasma gamma-glutamyl transpeptidase and alanine aminotransferase activity was increased by bile-duct ligation, and that L-aginine treatment partially, but significantly, prevented the elevation of these markers of liver damage. The results showed that heart rate of cholestatic animals was significantly less than that of sham-operated control rats in vivo and this bradycardia was corrected with daily administration of L-NNA. The basal spontaneous beating rate of atria in cholestatic animals was not significantly different from that of sham-operated rats in vitro. Meanwhile, cholestasis induced a significant decrease in chronotropic effect of epinephrine. These effects were corrected by daily administration of L-NNA. Surprisingly L-arginine was as effective as L-NNA and increased the chronotropic effect of epinephrine in cholestatic rats but not in sham-operated animals. Systemic NOS inhibition corrected the decreased chronotropic response to adrenergic stimulation in cholestatic rats, and suggests an important role for NO in the pathophysiology of heart rate complications in cholestatic subjects. The opposite effect of chronic L-arginine administration in cholestasis and in control rats could be explained theoretically by an amelioration of cholestasis-induced liver damage by chronic L-arginine administration in bile duct-ligated rats.
Collapse
Affiliation(s)
- A Nahavandi
- Department of Physiology, Iran University of Medical Sciences, P.O. Box 14155-6183, Tehran, Iran.
| | | | | | | | | | | |
Collapse
|